Co-channel interference suppresssion by estimating the time of arrival (toa)

ABSTRACT

The invention provides improved suppression of co-channel interference (CCI) in a wireless cellular communications system where at least two base stations use the same transmit frequency. The method according to the invention uses the difference in synchronisation times between the home signal and the CCI signal to suppress the CCI signal. The method uses a modified known method of estimating time of arrival (TOA) to estimate sync positions for the CCI signal. This enables the use of interference cancelling methods in communications systems using un-synchronised base stations. Receiver performance is improved, which in turn improves data throughput and received speech quality, since interfering signals from base stations using the same frequency can be suppressed.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to a method and a system for cancelling orsuppressing co-channel interference (CCI) in a wireless cellularcommunications system including at least two base stations using thesame frequency, and to a mobile radio station such as a mobilecommunications device or a mobile telephone for use in such acommunications system. The invention will be useful in TDMA and FDMAsystems such as GSM and IS-136.

TECHNICAL BACKGROUND

[0002] In cellular wireless communications systems each base stationcovers a geographical area called a cell. Neighbouring cells usually usedifferent frequencies. Frequencies can be reused in different cells, andcells using the same frequency are usually separated geographically byat least one intermediate cell using a different frequency. A mobiletelephone or other mobile communications device in one cell, called thehome cell, will normally be served by the base station, called the homebase station, in the home cell. For two cells using the same frequencytheir mutual geographical distance may not always give sufficientattenuation of their signals to ensure that their signals do notinterfere with each other, especially for hot spot areas, wherefrequencies need to be re-used for capacity reasons. Consequently, itmay happen that, at some locations in the home cell, the mobiletelephone can receive signals from a remote base station in a remotecell using the same frequency. The remote base station handlesinformation or traffic, which is unrelated to the information or traffichandled by the home base station. When signals from two unrelated basestations using the same frequency are received by a mobile telephone,the signals from the remote base station may disturb or interfere withthe signals received from the home base station. This is an undesiredphenomenon called co-channel interference (CCI), and the remote basestation is called a co-channel interferer.

[0003] It is the object of the invention to cancel or suppressco-channel interference.

[0004] U.S. Pat. No. 5,515,378 discloses a method and an apparatus forincreasing the capacity and the quality of communication between remoteusers and a base station. Co-channel interference is suppressed by usinga directive transmitter antenna array directed to individual mobilestations.

[0005] WO 98/59443 discloses a method and a system in which co-channelbase station are provided with a time reference signal being asynchronizing signal or a time reference from another co-channel radiobase station.

[0006] U.S. Pat. No. 5,317,323 discloses a system for accuratelylocating a mobile station using the Global Positioning System (GPS).

[0007] In general, in the state of the art systems for interferencesuppression, co-channel interferers are demodulated and interferencesymbols are subtracted from the received symbols. Co-channel interfererscan be demodulated in several known ways. After the interference symbolshave been subtracted from the received symbols, the remaining symbolsare demodulated to extract the useful information from each burst orunit of transmitted signal.

PROBLEM TO BE SOLVED BY THE INVENTION

[0008] None of the known solutions are fully feasible in acommunications system using un-synchronised base stations, or in asystem where the relative timing of the co-channel interferers isunknown. This is crucial to interference suppression efficiency. If thisinformation is missing an extensive search for the pilot symbols ortraining sequences has to be carried out, which is a complex and powerconsuming operation. It is therefore the object of the invention toprovide a more efficient method of co-channel interference suppression.

SUMMARY OF THE INVENTION

[0009] The invention provides improved suppression of co-channelinterference in a wireless cellular communications system where at leasttwo base stations use the same transmit frequency. Co-channelinterference suppression requires knowledge of the desired channel, iethe transmissions from the home base station, and of the interferingco-channel for the co-channel interference suppression to be efficient.The mobile station knows the characteristics of the home channel, but inmost cases the interfering co-channel is unknown, and itscharacteristics will therefore have to be estimated. Relevanttransmission channel characteristics include in particular the channelimpulse response and the relative time difference between the desiredsignal from the home base station and the signal from the interferingco-channel. According to the invention the relative time differencebetween the desired signal and the signal from the interferingco-channel is estimated by finding their synchronisation (short: sync.)positions, which is advantageously done by estimating their times ofarrival, TOA.

[0010] The method according to the invention uses a modified knownmethod of estimating time of arrival (TOA), which is already used forpositioning, to estimate channel characteristics, in particular impulseresponse and sync. position. This enables the use of interferencecancelling methods in communications systems using un-synchronised basestations. Receiver performance is improved, which in turn improves datathroughput and received speech quality, since interfering signals frombase stations using the same frequency can be suppressed. Alternatively,the capacity of the system can be increased by higher frequency reusewithout sacrificing data throughput or speech quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows the cell structure of a FDMA wireless communicationssystem,

[0012]FIG. 2 is a schematical block diagram of a digital receiver usingTOA assisted interference suppression,

[0013]FIG. 3 is a schematical flow chart showing the steps ofinterference suppression in accordance with the invention, and

[0014]FIG. 4 shows schematically the relative timing of a receiveddesired signal and a received interfering signal.

DETAILED DESCRIPTION OF THE INVENTION

[0015]FIG. 1 shows a home cell A in which the home base station uses thefrequency f₀. A number of neighbouring cells (in this case six areshown) cover the geographical area around the home cell A, and the basestations in neighbouring cells use different frequencies f₁ to f₆, allof which in particular are also different from the frequency f₀ used inthe home cell A. Further away from the home cell A and beyond thesurrounding neighbouring cells is a remote cell B in which the remotebase station uses the same frequency f₀ as is used in the home cell A.This is called frequency reuse. The remote base station in the remotecell B may thus cause co-channel interference to mobile stations in thehome cell A.

[0016] Transmitted data are divided into bursts or short packets ofdata. Each burst includes a training sequence, which is a predefinedsequence of bits occupying a predefined position in each burst. In thereceiver the training sequence will be recognised as such and will beused as a synchronising signal identifying eg the start and stoppositions of each burst. Base stations using the same transmit frequencyuse different training sequences. In addition to using differenttransmission frequencies neighbouring base stations may also usedifferent training sequences.

[0017]FIG. 4 shows a received desired signal from the home base stationand a received interfering signal CCI from a remote base station. Eachsignal consists of a series of bursts, where each burst contains twodata sequences with a training sequence TS between the two datasequences. The time difference between the two signals is indicated asthe time interval τ between the start of their respective trainingsequences.

[0018] The impulse response is the received signal that would resultfrom a short impulse being transmitted from the transmitter. In practiceseveral reflected signals will be received in addition to a directline-of-sight signal, which is known as multi-path transmission. In amulti-path environment the impulse response will therefore have several“peaks” separated in time and with different amplitudes corresponding tothe direct signal and several reflected signals being received withdifferent delays. In particular, estimates of the impulse response andsync. position or timing of the co-channel are required.

[0019] In a communications system where the receive or downlinkfrequency is reused in two or more cells the frequency reuse mayintroduce interfering signals or noise in the serving cell or home cellfrom the remote base station in a neighbouring or remote cell using thesame frequency. As a result the signals received by the mobile stationin the home cell will contain information from other base stations aswell as noise. With one co-channel interferer the received signal y(t)can be expressed mathematically as follows: $\begin{matrix}{{y(t)} = {{{\sum\limits_{k = 0}^{L}{h_{k}u_{t - k}}} + {\sum\limits_{k = 0}^{L}{g_{k}v_{t - k}}} + {e(t)}} = {{H^{T}U_{t}} + {G^{T}V_{t}} + e_{t}}}} & (1)\end{matrix}$

[0020] where

[0021] H^(T)=[h₀, . . . , h_(L)]^(T) is a complex-valued vectorrepresenting the radio channel of the home cell, U_(t)=[u_(t), . . . ,u_(t−L)]^(T) is a complex-valued vector representing the transmittedsymbols in the home cell,

[0022] G^(T)=[g₀, . . . , g_(L)]^(T) is a complex-valued vectorrepresenting the radio channel for the CCI,

[0023] V_(t)=[v_(t), . . . , v_(t−L)]^(T) is a complex-valued vectorrepresenting the transmitted symbols of the CCI, and

[0024] e_(t) is noise, which is often assumed to be white noise.

[0025] For the purpose of determining the geographical position of amobile station it is known to determine the time of arrival (TOA) ofsignals received at the mobile station. In such methods the mobilestation receives information from neighbouring base stations especiallysuitable for determination of position. In practice, and for capacityreasons, the TOA estimates are made for neighbouring base stations thatuse adjacent frequencies.

[0026] Co-channel interference suppression requires knowledge of thedesired channel, ie the transmissions from the home base station, and ofthe interfering co-channel for the co-channel interference suppressionto be efficient. The mobile station knows the characteristics of thehome channel, but in most cases the interfering co-channel is unknown,and its characteristics will therefore have to be estimated. Relevanttransmission channel characteristics include in particular the channelimpulse response and the relative time difference between the desiredsignal from the home base station and the signal from the interferingco-channel. According to the invention the relative time differencebetween the desired signal and the signal from the interferingco-channel is estimated by finding their sync. positions, which isadvantageously done by estimating their times of arrival, TOA.

[0027] Estimating time of arrival (TOA) can eg be carried out asfollows. Given a specific training sequence, collect estimates ofchannel characteristics for the current and previously received bursts,H_(vector)=[H_(t),H_(t+1), . . . , H_(t+N)], where each H_(k) is thechannel estimate for burst k and consists of complex-valued channel tapsH_(k)=[h_(k,0), k_(k,1), . . . h_(k,L)]. For each tap |{overscore (H)}|²is calculated: $\begin{matrix}\begin{matrix}{{\overset{\_}{H}}^{2} = \left\lbrack {{\sum\limits_{i = 0}^{N}{H_{i,0}}^{2}},{\sum\limits_{i = 0}^{N}{H_{i,1}}^{2}},\ldots \quad,{\sum\limits_{i = 0}^{N}{H_{i,L}}^{2}}} \right\rbrack} \\{= \left\lbrack {{{\overset{\_}{H}}_{0}}^{2},{{\overset{\_}{H}}_{1}}^{2},\ldots \quad,{{\overset{\_}{H}}_{L}}^{2}} \right\rbrack}\end{matrix} & (2)\end{matrix}$

[0028] The TOA estimate is used as an estimate of the sync. position,and is calculated as the position, i, for which the energy

E=|{overscore (H)} _(t)|²; iε[0,L]  (3)

[0029] is maximised. The references [1] and [2] give more details onthis.

[0030] The timing of a co-channel interferer can be obtained bymodifying the above method used in TOA measurements. Originally, TOAmeasurements were used for positioning purposes and for estimating thesynchronisation position for a list of base stations, whose signals andrelated training sequences are received by the mobile station. For eachbase station in the list a TOA estimate, ie an estimate of the timing ofthe base station, is made. Together with geographical information theTOA is then used to calculate the position of the mobile stationrelative to the base stations. The list of base stations is selectedsuch that the TOA estimates will produce a reliable result in terms ofpositioning performance. In a typical operation scenario the listincludes neighbouring base stations using other frequencies than thebase station home in the home cell.

[0031] The desired signal A from the home base station is usually muchstronger than the interfering signal from the co-channel. The sync.position of the desired signal is therefore calculated or estimatedusing only a short time correlation, typically only over one burst. Forthis purpose known standard correlation methods can be used.

[0032] Due mainly to the distance from the remote base station theinterfering signal from the co-channel is usually a weak signal. Inorder to obtain a proper estimate of the timing of the interferingsignal it is therefore necessary to use a longer time correlation,usually over many bursts. This is used in the invention

[0033]FIGS. 2 and 3 illustrate the invention. According to the inventiona TOA calculation is switched on, performed during normal reception, andused for estimating the timing and sync. position of the co-channelinterfering base station. In cases where the training sequence of theworst co-channel interferer is unknown, the average channel impulseresponse according to equation (2) above has to be carried out for eachof the training sequences used by the co-channel interferer. Selectionof a TOA estimate τ and which training sequence to use is then madejointly according to the criterion in equation (3) above.

[0034] When accurate timing information of the co-channel interferer isthus obtained the timing information is fed to a Sync. and Channelestimation unit that uses the timing information to calculate thecurrent channel impulse response for the CCI. This information is thenused in an equaliser for interference suppression that removes orsuppresses the CCI signals.

[0035] In case of lack of information on the CCI training sequence,which is provided by the network, a search over all possible pilotsequences will have to be carried out.

REFERENCES

[0036] [1] H. Murata, S. Yoshida: Maximum-Likelihood Sequence Estimationfor Coded Modulation in Presence of Co-Channel Interference andIntersymbol Interference, 1996 IEEE 46th Vehicular TechnologyConference, IEEE, New York, USA, 1996, 3rd vol xxxix+1887 pp 701-705vol.2.

[0037] [2] A. Nilsson-Stig, H. Perbeck: Equalization of Co-ChannelInterference in Future Mobile Communication Systems. Master thesis, LundInstitute of Technology, Sweden, 1999.

1. A method of suppressing co-channel interference in a mobile stationwhen used in a wireless cellular communications system including atleast a home base station and a remote base station both using the sametransmit frequency, the method comprising the following steps: receivinga signal transmitted from the home base station, setting a receivefrequency of the mobile station to the transmit frequency of the homebase station, and, if a signal from a remote base station is received,performing the following steps: determining a synchronisation time forthe received signal from the home base station, determining asynchronisation time for the received signal from the remote basestation, determining the time difference between the synchronisationtime for the received signal from the home base station and thesynchronisation time for the received signal from the remote basestation, and using the time difference in a process suppressing thereceived signal from the remote base station.
 2. A method according toclaim 1 wherein the synchronisation times for the received signals aredetermined by estimating their respective times of arrival at the mobilestation.
 3. A method according to claim 2 wherein the synchronisationtime for the received signal from the home base station is determinedusing a short-term correlation over the signal from the home basestaiton.
 4. A method according to claim 3 wherein the estimation of timeof arrival involves detecting predefined known training sequences ofdata in the signal from the home base station.
 5. A method according toclaim 2 wherein the synchronisation time for the received signal fromthe remote base station is determined using a long-term correlation overthe signal from the remote base station.
 6. A method according to claim5 wherein the estimation of time of arrival involves detectingpredefined known training sequences of data in the signal from theremote base station.
 7. A mobile station for use in a wireless cellularcommunications system including at least two base stations using thesame frequency the mobile station comprising means for performing amethod according to claim 1.